Protonation equilibria studies of quercetin in aqueous solutions of ethanol and dimethyl sulphoxide

The protonation constants of quercetin were determined in different aqueous solutions of ethanol and dimethyl sulphoxide using a combination of potentiometric and spectrophotometric methods at constant temperature (25.0 degrees C) and ionic strength (0.1 mol dm(-3) NaCl). It has been shown that the protonation constants of quercetin increases with increasing proportion of the both organic solvents in the binary mixtures. The protonation constants were analyzed using dielectric constant and Kamlet, Abboud, and Taft (KAT) parameters. A linear correlation of log K versus the reverse dielectric constant was obtained in the different aqueous organic solvents solutions. Dual-parameter correlation of log K versus a (hydrogen bond donor acidity) and pi* (dipolarity/polarizability) parameters gave good results in the different binary aqueous solutions of the organic solvents. Three different extrapolation procedures including mole fraction, reversed dielectric constant, and Yasuda-Shedlovsky plots were used to determine the protonation constants of quercetin at zero percent organic solvent. Finally, the results were discussed in terms of the effect of organic solvent on protonation equilibria. (C) 2016 Elsevier B.V. All rights reserved.